Lake Water Clarity: Minnesota Statewide
Water clarity, an indicator of water quality, is a loosely defined term generally related to how far one can see in a water body. For background information on measuring water clarity click here.
Using Landsat satellite imagery, we have been using procedures explained in the “Methods” page to measure water clarity of all lakes 8 hectares (20 acres) or larger in the state of Minnesota. This geographic scale requires the processing and classification of 19 Landsat satellite images. Thus far we have completed nine complete mappings (or censuses) of Minnesota lake water clarity for the nominal year 1975, 1985, 1990, 1995, 2000, 2005, 2008, 2010, and 2015. The classifications at approximately five-year intervals from 1975 to 2015 provide an unprecedented assessment of lakes in terms of the number of lakes and geographic and temporal extent. Secchi depth (SD) measurements collected within a few days (usually ±3 days) of each Landsat image were obtained from the Citizen Lake Monitoring Program (CLMP). relies on citizen volunteers who measure SD on ~900-1100 Minnesota lakes (the exact number varies over the years) multiple times during the open-water season. CLMP is managed by the Minnesota Pollution Control Agency, which maintains the SD database.
The models described on the background information page generally yielded R2 values in the range 0.80-0.90. Click on the above map to see the nine classifications from 1975 to 2015. This work was published (Olmanson et al. 2008) in the journal, Remote Sensing of Environment. If you are interested in more detailed information, including temporal trends in clarity for individual lakes, please visit the LakeBrowser.
We analyzed the 1985-2008 data for temporal and geographic patterns and trends, and relationships to land use and other factors that may cause changes in lake quality. Key
results (Olmanson et al., 2013) from that analysis are shown in the figure at the right and below. From 1985 to 2005, mean water clarity at the state level remained relatively stable; 4.6% of lakes had increased clarity and 6.2% decreased, but there were large differences in temporal trends among the ecoregions. As shown in the figure at the left below, there also are strong geographic patterns in water clarity, with lower clarity in the south and higher clarity in the north. In addition, as the figure at the right below shows, deeper lakes tend to have higher clarity and are more stable than shallow lakes, and agricultural and urban land use are associated with lower clarity.
Olmanson, L. G., M. E. Bauer and P. L. Brezonik. 2008. Development and analysis of a 20-year Landsat water clarity census of Minnesota’s 10,000 lakes. Remote Sens. Environ. 112: 4086-4097.
Olmanson, L. G., P. L. Brezonik, and M. E. Bauer. 2013. Geospatial and temporal analysis of a 20-year record of Landsat-based water clarity in Minnesota’s 10,000 lakes. J. Amer. Water Resour. Assoc. 50(3): 748-761.